Links Between Short - Term Beach Evolution and Resulting Heterogeneities in Wave - Dominated , Upper - Shoreface - Foreshore Sandstone Bodies

ISLA M.; VEIGA G.D.; SCHWARZ E.

Salt Lake City

Congreso; AAPG 2018 Annual Convention & Exhibition (ACE); 2018

AAPG

Despite the knowledge on morphodynamics of regressive, wave-dominated barrier/strandplain systems,processes responsible for sand accretion and seaward migration operating at 102-103 year scale in theupper-shoreface and foreshore regions are far from been understood. In this contribution we analyzearchitectural styles of foreshore-to-upper-shoreface sand bodies of microtidal, barrier/strandplain systems, inorder to reconstruct depositional processes and discuss preservation of morphological features, and possiblecontrolling factors. Selected cases studies include Cainozoic and Cretaceous examples with a narrow grainsize range (very fine to medium sand), in which facies and architectural styles allow for reconstructions ongradients and morphological configurations.Three main architectural styles were recognized. Type 1 and Type 2 styles are characterized by preservationof upper-shoreface (US), cross-bedded sandstones and foreshore (FS) parallel planar laminated, but withcontrasting depositional gradients. Type 1 shows FS deposits downlapping (seaward) onto upper-shorefacestrata with a clear slope break (> 5° to < 1°), whereas Type 2 shows subhorizontal transitions between FSand US deposits. Type 3 style is characterized by large-scale, seaward dipping foreset beds (11°-2°) with atangential geometry, and weak discrimination between foreshore and upper-shoreface sediments. Parallelplanar lamination and small-scale trough cross-stratification are common, the latter becoming dominant at thebottomsets.Types 1 and 2 are interpreted to represent long-lived, bar-trough systems in gentle to moderate beachprofiles, whereas Type 3 reflects bar-less, steeper beachfaces. Bar-related deposits (e.g., swash orlongshore bars) are uncommon, suggesting low preservation of these features in regressive, microtidalsystems. The analyzed dataset suggests that the generation and preservation of any single architectural styleseems unrelated to the wave-dominated system type or shoreline trajectory. In contrast, they likely reflectpreferential sediment accumulation in the US or FS settings over 102-1003-year periods, which eventuallycould be linked to the associated beach profiles (from dissipative to reflective beaches).The identification of different architectural styles in these US-FS deposits as a result of contrasting mid-termevolution of the wave-dominated, regressive system may provide an innovative insight for thecharacterization of paralic reservoirs.